How do I mix more than one image with a video in node editor?
To mix more than one image you concatenate nodes: the result of one mix node gets connected to the next node as input.

For example:



It is important to know what kind of operation you want to use to mix the images. (read the manual page for the mix node for more info)

If you are mixing images with alpha channel, use the Alpha Over node instead. Read this related link: My objects are semi-transparent, and not one behind the other in the compositor